Phil, I've seen this recently.
In my case it seemed to be linked to a network outage. The odd thing was the outage was only for an hour or two but it took hours longer for the workflows to recover (on their own) and be able to retrieve variables from the Process Manager. It has made me shy away from using PM based variables even though they are perfect for information that is shared by many workflows. I have been using OS based environment variables as a workable alternative. Not so good if you're running a clustered setup though.